Method for heating up catalysts in the exhaust gas of internal combustion engines

ABSTRACT

A heating of a catalytic converter in the exhaust gas of an internal combustion engine is presented which can be operated in different operating modes and wherein at least one of several heating measures can be selected so that, at first, an estimate can be made for several heating measures as to whether an individual heating measure can make the desired heating effect available and that it is further estimated whether an individual heating method can be carried out in the instantaneous operating state with a view to the exhaust-gas values and the operating mode of the internal combustion engine, which is necessary for carrying out the heating measure; and that that operating mode is requested wherein the requests can best be satisfied and that at least one possible heating measure is activated in dependence upon the instantaneous mode of operation. The invention further is directed to an electronic control unit for carrying out the method.

[0001] Catalytic converters in the exhaust gas of internal combustionengines need a specific minimum temperature (light off temperature) inorder to develop their toxic substance converting operation. Thistemperature is to be reached as rapidly as possible after a cold start.In engines, which are operated with a lean air/fuel mixture (forexample, in engines having gasoline-direct injection and nitrogen oxidestorage catalytic converters), additional requirements, which are inpart changing requirements, are imposed on the catalytic convertertemperature, for example, because of a necessary desulfatization of thestorage catalytic converter during driving operation. A desulfatizationrequires, for example, temporarily a higher catalytic convertertemperature is necessary than during normal operation for the storage ofthe nitrogen oxides.

[0002] Measures are already known for heating catalytic converters. Forexample, the engine combustion can take place with so rich a mixturethat the exhaust gas still contains uncombusted fuel. The supply ofsecondary air to the exhaust gas causes a reaction-capable mixture toform which heats up the catalytic converter via an exothermal reaction.

[0003] Furthermore, the engine combustion can take place with so lean amixture that the exhaust gas still contains uncombusted oxygen. In thiscase, a reaction-capable mixture can be generated by the metering offuel to the exhaust gas.

[0004] It is further known to heat up the catalytic converter via theconsequences of a deterioration of the efficiency of the enginecombustion. A deterioration of efficiency of the engine combustion can,for example, be introduced by a deviation of the ignition time pointfrom the optimal time point. The optimal time point is defined by themaximum efficiency. Because of the reduction of efficiency, the exhaustgas is hotter in comparison to the operation without a loss inefficiency. Accordingly, the exhaust gas develops an increased heatingin the catalytic converter.

[0005] In engines having gasoline-direct injection, different modes ofoperation of the engine permit different measures for heating thecatalytic converter.

[0006] An engine control program is known from DE 198 50 586 whichcontrols the switchover between stratified operation and homogeneousoperation.

[0007] In stratified operation, the engine is operated with an intenselystratified cylinder charge and high air excess in order to achieve thelowest possible fuel consumption. The stratified charge is achieved viaa late fuel injection which, in the ideal case, leads to a partitioningof the combustion chamber into two zones: the first zone contains acombustible air/fuel mixture cloud at the spark plug. The first zone issurrounded by the second zone and this second zone comprises aninsulating layer of air and residual gas. The potential for optimizingconsumption results from the possibility of operating the enginesubstantially unthrottled while avoiding charge exchange losses. Thestratified operation is preferred at comparatively low loads.

[0008] At higher load, when the power optimization is primary, theengine is operated with a homogeneous cylinder charge. The homogeneouscylinder charge results from an early fuel injection during theinduction operation. As a consequence, a longer time up to thecombustion is available for mixture formation. The potential of thisoperating mode for power optimization results, for example, fromutilizing the entire combustion chamber volume for filling with acombustible mixture.

[0009] An exhaust-gas composition can be adjusted for heating anNOx-storage catalytic converter in homogeneous operation with thisexhaust-gas composition deviating from the stoichiometric exhaust-gascomposition.

[0010] In gasoline-direct injection engines, the possibility is furtherprovided of targetly injecting fuel into the cylinder in the expansionstroke after the engine combustion when operation is with air excess,that is, preferably, in stratified operation. Here, the after-injectedfuel partially reacts with the air excess of the engine combustionpartially already in the combustion chamber and partially in theexhaust-gas system. The heat, which is released by the exothermalreaction, heats the catalytic converter.

[0011] The task of the invention is to select an optimal heatingstrategy in each operating state.

[0012] This task is solved with the features of claim 1.

[0013] The heating of a catalytic converter in accordance with theinvention takes place in the exhaust gas of an internal combustionengine which can be operated in different operating modes and wherein atleast one of several heating measures can be selected so that at first,for several heating measures, an estimate is made as to whether anindividual heating measure can make available the wanted heating effectand that it is further estimated whether an individual heating measurecan be carried out in the actual operating state with a view to theexhaust-gas values and the operating mode of the engine necessary forcarrying out the heating measure and that that operating state isrequested in which the requirements can be best satisfied and that atleast one possible heating measure is activated in dependence upon theinstantaneous mode of operation. The instantaneous operating state ischaracterized, for example, by values for the catalytic convertertemperature, the vehicle speed and the instantaneous load.

[0014] One embodiment is characterized in that a deterioration in theefficiency of the engine combustion takes place as a function of achange of the ignition angle as one measure.

[0015] A further measure provides that, as a second measure, a fuelafter-injection takes place after the combustion in an engine havinggasoline-direct injection.

[0016] A further measure provides that the after-injection is combinedwith stratified operation.

[0017] Another measure provides that the air quantity, which is inductedby the engine, is throttled to the extent that the necessary heat flowis achieved at a requested temperature.

[0018] A further measure provides that an exhaust-gas composition isadjusted in homogeneous operation for heating an NOx-storage catalyticconverter with this exhaust-gas composition deviating from thestoichiometric exhaust-gas composition.

[0019] The invention is also directed to an electronic control unit forcarrying out the measures and method steps.

[0020] The various operating modes of the internal combustion enginehaving gasoline-direct injection permit different measures for heatingthe catalytic converter. The allocation of the invention of heatingmeasures and operating modes makes possible an optimization of theheating strategy with a view to the operating state of the vehicle,which, for example, is determined by parameters such as catalyticconverter temperature, vehicle speed and torque requirement.

[0021] Advantageously, the possible heating effects of various catalyticconverter heating measures are estimated and compared to the heatingaction need. The heating action need for heating a catalytic converterleads, for example, to physical requests as to the quantity and thetemperature of the exhaust-gas flow which must be made available via theheating measure.

[0022] Furthermore, the operating limits for the individual modes ofoperation are considered. In this way, it is possible to select anoptimal heating strategy in each operating mode.

[0023] All requirements for active heating measures are formulated asheat flow requests and temperature requests in the exhaust gas. In thisway, all requests can be handled in a uniform manner.

[0024] In the following, an embodiment of the invention is explainedwith respect to the figures. FIG. 1 shows the technical background ofthe invention. FIG. 2 shows an embodiment of the invention in the formof a flow diagram.

[0025] In FIG. 1, 1 represents the combustion chamber of a cylinder ofan internal combustion engine. The flow of air to the combustion chamberis controlled via an inlet valve 2. The air is drawn in by suction viaan intake manifold 3. The intake air quantity can be varied via athrottle flap 4 which is driven by a control apparatus 5. The followingare supplied to the control apparatus: signals as to the torque commandof the driver (for example, via the position of an accelerator pedal 6);a signal as to the engine rpm n from an rpm transducer 7; a signal as tothe quantity ml of the inducted air by an air quantity sensor 8; and, asignal US as to the exhaust-gas composition and/or exhaust-gastemperature from an exhaust-gas sensor 12. The exhaust-gas sensor 12can, for example, be a lambda probe whose Nernst voltage indicates theoxygen content in the exhaust gas and whose internal resistance can beapplied as an index for the probe temperature, exhaust-gas temperatureand/or catalytic converter temperature. The exhaust gas is conductedthrough at least one catalytic converter 15 as part of an exhaust-gassystem 16 wherein toxic substances of the exhaust gas are convertedand/or are temporarily stored (NOx-storage catalytic converter).

[0026] The control apparatus 5 forms output signals for adjusting thethrottle flap angle (α) via an actuating member 9 and for driving a fuelinjection valve 10 via which fuel is metered into the combustionchamber. The control apparatus 5 forms these output signals from theabove, and, if required, additional input signals as to other parametersof the internal combustion engine such as intake air temperature andcoolant temperature, et cetera. Furthermore, a triggering of theignition via an ignition device 11 is controlled by the controlapparatus.

[0027] The throttle flap angle (α) and the injection pulse-width (ti)are essential actuating variables, which are to be matched to eachother, for realizing the desired torque, the exhaust-gas composition andthe exhaust-gas temperature. A further significant actuating variablefor influencing these variables is the angular position of the ignitionrelative to the piston movement. The determination of the actuatingvariables for adjusting the torque is the subject matter of DE 198 51990 which is to be incorporated into the disclosure.

[0028] Furthermore, the control apparatus controls additional functionsfor achieving an efficient combustion of the air/fuel mixture in thecombustion chamber, for example, an exhaust-gas recirculation (notshown) and/or tank venting. The gas force, which results from thecombustion, is converted into a torque by the piston 13 and thecrankshaft 14.

[0029] In this technical background, the catalytic converter temperaturecan be measured or can be modeled from operating variables of theengine. The modeling of temperatures in the exhaust-gas system ofinternal combustion engines is known, for example, from U.S. Pat. No.5,590,521.

[0030] For heating by means of after-injection, the engine controlaccording to the invention requires minimum temperatures in theexhaust-gas system. Until these temperatures are reached, for example,homogeneous operation with retarded ignition is requested as a firstmeasure and adjusted. When the necessary temperatures are reached, theafter-injection is permitted as a possible alternative. The switchoverto stratified operation with after-injection takes place in order togenerate a higher heat flow. The air flow is then throttled to theextent that the needed heat flow is reached at a requested temperature.

[0031] The throttling takes place in a first embodiment via anadjustable closing of the throttle flap by a predetermined angle or to apredetermined opening angle. Stated otherwise, the throttling takesplace uncontrolled in this example. The mixture composition should beclose to lambda equals 1 for a maximum release of heat. Temporarymixture enrichment to lambda values less than 1 can occur because of adynamic driving operation with changing torque requirements. In thisway, the exhaust-gas emissions are deteriorated in an unwanted manner.

[0032] To avoid an exhaust-gas deterioration, the after-injection isadvantageously controlled with the aid of an exhaust-gas probe which ispresent. In this way, a breakthrough of rich exhaust gas can beprevented. The breakthrough is characterized by the occurrence of HCemissions rearward of the catalytic converter. As a further advantage,the exothermal energy release at lambda equals 1 is maximally utilized.

[0033] In detail, because of the heat request, a necessary fuel quantityis determined for the after-injection at maximum possible throttling. Inaddition to the heat request, also the air requirement of theafter-injection and the temperature increase because of the throttlinghave to be considered. The latter is especially important to preventoverheating of components in the exhaust-gas system.

[0034] The throttling can be controlled via the measured exhaust-gaslambda as an alternative to the control of the after-injected fuelquantity via the measured exhaust-gas lambda.

[0035]FIG. 2 shows an embodiment in the form of a flow diagram.

[0036] In step 2.1, a check is made as to whether a request for acatalytic converter heating measure is present. If this is the case,then, in step 2.2, an estimate for the at least one heating measure Xtakes place as to whether this measure can make the wanted heat effectavailable and whether this heating measure is permitted with a view tothe exhaust-gas values and the operating mode of the internal combustionengine, which is necessary for carrying out the heating measure, in theinstantaneous operating state (exhaust-gas system temperature and/orcatalytic converter temperature, vehicle speed, instantaneous load).

[0037] The various heating measures are computed and evaluated based onphysical requirements (heat flow and temperature). Furthermore, theoperating limits for the individual operating modes are considered. Inthis way, it is possible to select an optimal heating strategy for eachoperating state. Furthermore, a check is made for each heating measureas to whether the heating measure, with a view to the exhaust-gas valuesand the required operating mode, is possible in the instantaneousdriving state, that is, for the instantaneous catalytic convertertemperature, vehicle speed and load.

[0038] With this information, the individual heating measures can beevaluated in dependence upon the operating state and a decision can bemade for the best measure. The evaluation of a heating measure takesplace with the present request in a slow time raster for each heatingmeasure. The necessary interventions are computed in a rapid time rasteronly for the activated heating measure in order to save computationtime.

[0039] If necessary, the request for the required optimal operating modetakes place in step 2.3. For example, after-injection in the stratifiedoperation cannot be optimal for a cold exhaust-gas system because in anexhaust-gas system which is too cold, the necessary after-reaction ofthe resulting mixture in the exhaust gas cannot take place. Catalyticconverters which are too cold can especially not support theafter-reaction in the exhaust-gas system. In this case, a deteriorationof efficiency because of retarded ignition would be appropriate toincrease the exhaust-gas temperature. This can be preferably carried outin the operating mode with homogenous operation. Correspondingly, instep 2.3, a switchover can take place into the homogeneous operation.

[0040] Thereafter, the activation of the selected heating measure takesplace in step 2.4.

[0041] If, in contrast, the inquiry in step 2.1 is negative, then norequest of a heating measure takes place and possibly activated heatingmeasures are deactivated.

1. Method for heating a catalytic converter in the exhaust gas of an internal combustion engine, which can be operated in different operating modes and wherein at least one of several heating measures can be selected; wherein an estimate is made for several heating measures as to whether an individual heating measure can provide the wanted heating effect; and wherein an estimation is made as to whether an individual heating measure can be carried out in the instantaneous operating state (temperatures in the exhaust-gas system, vehicle speed, instantaneous load) with a view to the exhaust-gas values and the operating mode of the internal combustion engine, which is necessary for carrying out the heating measure; and wherein the operating mode is requested wherein the requests can be best satisfied; and wherein at least one possible heating measure is activated in each case in dependence upon the instantaneous operating mode.
 2. Method of claim 1, characterized in that, as a measure, a deterioration of the efficiency of the engine combustion takes place via a change of the ignition angle.
 3. Method of claim 1, characterized in that, as a further measure in an engine having gasoline-direct injection, a fuel after-injection takes place after the combustion.
 4. Method of claim 3, characterized in that the after-injection is combined with stratified operation.
 5. Method of claim 4, characterized in that the air quantity, which is inducted by the engine, is throttled to the extent that the needed heat flow is reached with a requested temperature.
 6. Method of claim 1, characterized in that an exhaust-gas composition is adjusted for the heating of an NOx-storage catalytic converter in homogeneous operation, the exhaust-gas composition deviating from the stoichiometric exhaust-gas composition.
 7. Electronic control unit for carrying out the method of at least one of the claims 1 to
 6. 